1. Field of the Invention
The present invention relates to a surface acoustic wave filter to be used for a high frequency wave circuit or the like in, for example, a radiocommunication apparatus.
2. Related Art of the Invention
The electromechanical function parts using surface acoustic wave (SAW) have been noted with current attention in the hardware high density because the acoustic velocity of the wave is several kilometers/second, and the wave energy has properties to be concentrated on the surface of the propagation medium. Due to the development of the interdigital transducer (IDT) electrode and progress of the thin film preparing technique which has made its modified development possible, the same has been practically utilized for delay line for radar, band-pass filters for television receiver, etc. At present, the SAW is extensively used as RF and IF stage filters for receiving and transmitting circuits in radiocommunication apparatus.
In recent years, in consequence of the adoption of digital systems for moving objects, development of digital portable telephone and digital cordless telephone is intense. As the communication systems to be used for these apparatuses have information on the amplitude and phase of signals, the flatness of amplitude characteristics and group delay deflection characteristics are required for the filters to be used for IF stage. Also, as the excellent characteristic is required for the selectivity to distinguish the signal of an adjacent channel and the desired signal an acute attenuation characteristic having narrow transition bandwidth is also an essential condition. Also, recently, balanced input and output of the IC device in the stage before and after the IF filter have progressed, and the balanced input and output are required for the IF filter.
Conventionally, as a SAW filter suitable for the IF stage there are known a transversal SAW filter and two kinds of longitudinal mode coupled and transverse mode coupled type SAW filters. The transversal SAW filter has an excellent group delay deflection characteristic, but it has large insertion loss, poor attenuation characteristic, and large element size. On the other hand, the mode coupled type SAW filter presents an acute attenuation characteristic, shows small insertion loss, and is small in element size, but its group delay deflection characteristic is inferior to that of a transversal type SAW filter. The longitudinal mode type SAW is characterized by having a relatively large spurious zone on the high band side in the vicinity of the passing band, and the transverse mode type SAW filter is characterized by having a very narrow band characteristic. In view of the above characteristics, as the IF filter for the mobile communication apparatus the transverse mode coupled type SAW filter which is miniature in size and excellent in attenuation characteristic has been widely used.
Hereinafter, explanation is made on the conventional transverse mode coupled type SAW filter.
FIG. 24 is a constitution view showing a transverse mode coupled resonator type SAW filter according to conventional technique. In FIG. 24, the part 241 is a single crystal piezoelectric substrate. By forming an electrode pattern on the piezoelectric substrate 241, the SAW can be excited. The part 242a is an IDT electrode formed on the piezoelectric substrate 241, and by setting the reflector electrodes 242b and 242c on both sides thereof, an energy sealing in type SAW resonator is formed. On the piezoelectric substrate 241, there is formed a similar SAW resonator by the IDT electrode 243a and the reflector electrodes 243b and 243c. And, these two resonators are disposed nearby, and because of the formation of an acoustic couple between them, the SAW filter is constituted.
In the SAW filter constituted as above, two kinds of SAW mode frequencies to be excited on the piezoelectric substrate are determined by the electrode finger crossing width of IDT electrodes and the distance between the two SAW resonators disposed nearby, and the passing band width of the filter is determined.
In the SAW filter constituted as above, the bandwidth that can be realized is very narrow, and the specific bandwidth of the filter to be realized (normalized bandwidth at the central frequency of the filter) is at most about 0.1%. In order to meet the recent digital system, it is required to make the filter passing characteristics of wider bandwidth and broaden the flat bandwidth of the group delay deflection characteristic.
Also, recently a balanced input and output of IC device in the pre- and post-stages of IF filter have progressed. Accordingly, a balanced input and output type is strongly demanded for the IF filters. However, as shown in FIG. 24, in the conventional SAW filters, the one side of the electrode fingers of input and output stages of the IDT electrodes 242a, 243a is grounded, and there is a problem that the filter cannot be formed in a balanced input and output type.
Furthermore, there has been desired the impedance matching between the IF filter and the IC devices in the pre- and post-stages thereof, and as the input and output impedance's of the conventional filters depend on the number of pairs of the electrode fingers included in the IDT electrodes which are closely related with the filter characteristic, there has been a problem of it being difficult to obtain the desired impedance value simultaneously with obtaining the desired filter characteristic.